1. Field of the Invention
The present invention relates to the field of radio navigation and, more particularly, to radio navigation using radio transmitters.
2. General Background
Global positioning systems (GPS) have become nearly ubiquitous as navigational aids, for individual users as well as, more importantly, aviation users. GPS receivers are accurate and relatively inexpensive, and they are also easy to use, because they are area navigation (RNAV) systems. RNAV systems allow navigation on any desired course within the system's area of coverage. In other words, an RNAV user is not limited to following a specific track to or from a radio navigation aid (as a user must with some navigational aids), but can simply plot and follow waypoints that make up an RNAV route.
For aircraft navigation, GPS is currently used mainly as a supplemental RNAV system, although there are plans in the United States to make GPS the primary basis for radionavigation by the year 2010. There is a strong economic motivation to use GPS as a primary basis of navigation due to the expense of maintaining land-based navigational aids (navaids) currently in place, such as LORAN, VOR, DME, etc. For critical uses such as aircraft navigation, however, the very low power signals employed by GPS satellites can be a cause for concern, especially if GPS is used as a primary basis of navigation, rather than as a backup system for other established navigation systems. Specifically, the extremely low-power GPS signals may be susceptible to jamming or interference due to weather conditions, radio noise, or other sources. This is true even though GPS augmentations such as the Local Area Augmentation System, Wide Area Augmentation System, and other augmentations can improve the accuracy, integrity, and availability of the basic GPS.
Some navaids, such as automatic direction finding (ADF) and very-high frequency omni-directional range (VOR) are not automatic, provide only limited information, such as deviation from a given ground track, and can increase pilot workload because their use is non-intuitive. RNAV systems other than GPS require highly specialized airborne equipment and ground-based transmitters, and provide limited coverage of available airspace. Thus, an accurate, high-availability RNAV system whose infrastructure costs little or nothing to build and maintain, that is completely independent of GPS and traditional ground-based navaids, and that is automatic and intuitive to use would be desirable, and could greatly increase the safety and reliability of commercial and general aviation. In addition, having an RNAV system as a primary or backup navigation system (rather than ADF or VOR, for example) allows lateral freedom and thus a more complete and efficient use of airspace.
Furthermore, while GPS can provide a highly accurate positional fix for an aircraft (or other vehicle), it does not provide a true heading. This can create inconsistency in an aircraft heads-up display that relies on a GPS receiver as a primary basis of navigation. Specifically, as an airport, for example, becomes visible when an aircraft approaches it during a low-visibility condition, the heads-up display will not be aligned with the visible scene if the aircraft's true heading differs from its true bearing (for example, when strong crosswinds are present). Thus, an apparatus that provides a true heading could improve the functionality of an aircraft navigation system.